Exposure control apparatus and control method thereof, image capturing apparatus, and storage medium

ABSTRACT

An exposure control apparatus comprises an acquisition unit including a plurality of pixels that are arranged two-dimensionally, and configured to acquire image data, a compression unit configured to compress the image data and generate compressed data, a calculation unit configured to calculate a first photometric value based on the compressed data, a conversion unit configured to convert the first photometric value into a second photometric value corresponding to the image data before the compression; and an exposure control unit configured to perform exposure control based on the second photometric value.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an exposure control apparatus thatperforms photometry using a charge-accumulation type image sensor.

2. Description of the Related Art

A photometry range required of an image capturing apparatus such as adigital camera or digital video camera is generically about −5 to +15 ina BV value of the APEX unit. That is, the dynamic range of thephotometry range is about 20 steps. On the other hand, a dynamic rangethat can be expressed by one accumulation of a charge-accumulation typeimage sensor is about 10 steps.

There is known a technique of combining image signals obtained bycapturing the same scene a plurality of times under different exposurevalues using a charge-accumulation type image sensor, thereby generatingan image having a dynamic range wider than that implemented by oneaccumulation (image capturing). In general, this technique is called HDR(High Dynamic Range) combination.

For example, in Japanese Patent Laid-Open No. 6-130462, a photometricapparatus using a charge-accumulation type light-receiving elementalternately performs photometry with a long charge accumulation time andphotometry with a short charge accumulation time, thereby obtainingphotometric values from a low brightness portion to a high brightnessportion even if the brightness difference in the field is very large.

In Japanese Patent Laid-Open No. 2008-113029, two photoelectricconversion elements of different saturated exposure amounts are used,thereby expanding the dynamic range.

However, when photometric calculation is performed for an image with anexpanded dynamic range as described in Japanese Patent Laid-Open Nos.6-130462 and 2008-113029, the data amount of each pixel output becomeslarge. When performing integration processing and multiplicationprocessing using the pixel outputs, the circuit scale becomes large, andthe processing time is prolonged.

SUMMARY OF THE INVENTION

The present invention has been made in consideration of theabove-described problem, and can decrease various kinds of calculationamounts and perform appropriate photometric calculation when performingphotometric calculation using an image with an expanded dynamic range.

According to the first aspect of the present invention, there isprovided an exposure control apparatus comprising: an acquisition unitincluding a plurality of pixels that are arranged two-dimensionally, andconfigured to acquire image data; a compression unit configured tocompress the image data and generate compressed data; a calculation unitconfigured to calculate a first photometric value based on thecompressed data; a conversion unit configured to convert the firstphotometric value into a second photometric value corresponding to theimage data before the compression; and an exposure control unitconfigured to perform exposure control based on the second photometricvalue.

According to the second aspect of the present invention, there isprovided an image capturing apparatus comprising: an acquisition unitincluding a plurality of pixels that are arranged two-dimensionally, andconfigured to acquire image data; a compression unit configured tocompress the image data and generate compressed data; a calculation unitconfigured to calculate a first photometric value based on thecompressed data; a conversion unit configured to convert the firstphotometric value into a second photometric value corresponding to theimage data before the compression; an exposure control unit configuredto perform exposure control based on the second photometric value; andan image capturing unit configured to capture an object image underexposure control by the exposure control unit.

According to the third aspect of the present invention, there isprovided a control method of an exposure control apparatus including anacquisition unit including a plurality of pixels that are arrangedtwo-dimensionally, and configured to acquire image data, comprising:compressing the image data and generating compressed data; calculating afirst photometric value based on the compressed data; converting thefirst photometric value into a second photometric value corresponding tothe image data before the compression; and performing exposure controlbased on the second photometric value.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing the arrangement of an image capturingapparatus according to the first embodiment of the present invention;

FIG. 2 is a flowchart for explaining shooting processing according tothe first embodiment;

FIG. 3 is a flowchart for explaining photometric processing according tothe first embodiment;

FIGS. 4A to 4J are views showing an example of a shooting sceneaccording to the first embodiment;

FIG. 5 is a view showing the relationship between BV values and pixeloutputs of a 24-bit high dynamic range image;

FIG. 6 is a view showing the relationship between photometriccalculation pixel output and most significant bits;

FIG. 7 is a view showing the histograms of most significant bitsaccording to the first embodiment;

FIG. 8 is a view for explaining a bit shift amount deciding methodaccording to the first embodiment;

FIG. 9 is a view for explaining a method of compressing a pixel outputinto a predetermined data amount; and

FIG. 10 is a flowchart for explaining shooting processing according tothe second embodiment.

DESCRIPTION OF THE EMBODIMENTS

The embodiments of the present invention will now be described in detailwith reference to the accompanying drawings.

First Embodiment

FIG. 1 is a block diagram showing the arrangement of an image capturingapparatus according to the first embodiment of the present invention.Referring to FIG. 1, reference numeral 100 denotes a camera body; 200, aphotographing lens; and 300, a flash. Note that the image capturingapparatus may be a digital still camera, digital video camera,smartphone, tablet terminal, or the like having an arrangement differentfrom that shown in FIG. 1.

The arrangements of the camera body 100 and the photographing lens 200will be described first with reference to FIG. 1. In the camera body100, a CPU (to be referred to as a camera microcomputer hereinafter) 101is formed from a microcomputer that controls each unit of the camerabody 100. A memory 102 such as a RAM or ROM is connected to the cameramicrocomputer 101. An image sensor 103 is, for example, a CCD or CMOSsensor including an infrared cut filter, a low-pass filter, or the like.An object image is formed on the imaging plate of the image sensor 103by the photographing lens 200. A shutter 104 blocks light to the imagesensor 103 at the time of non-shooting, and opens to guide a light beamto the image sensor 103 at the time of shooting.

A half mirror 105 reflects some components of light that has enteredfrom the photographing lens 200 at the time of non-shooting, and formsan image on a focus plate 106. A photometric sensor 107 performs objectrecognition processing such as photometric processing, face detectioncalculation, or tracking processing using an image sensor such as a CCDor CMOS sensor in which pixels are two-dimensionally arranged.

Note that if a conventional sensor that is not a charge-accumulationtype area sensor is used as the photometric sensor, the dynamic range ofphotometry is 20 or more steps, meeting the dynamic range required ofthe image capturing apparatus. On the other hand, in the photometricsensor used in this embodiment, which uses a charge-accumulation typearea image sensor such as a CCD or CMOS sensor, the dynamic range isabout 10 steps, that is, narrower than that of the conventionalphotometric sensor. The area sensor having a narrow dynamic range isused as the photometric sensor because the image information of a fieldcan be obtained using an area sensor, and face detection processing orobject tracking processing can therefore be performed using only theoutput from the photometric sensor, as described above. In thisembodiment, high dynamic range processing (to be described later) isperformed for an image obtained by the photometric sensor 107 so as toensure a dynamic range necessary for the image capturing apparatus usingthe area type photometric sensor 107.

A pentaprism 108 guides the object image on the focus plate 106 to thephotometric sensor 107 and an optical viewfinder 109. The photometricsensor 107 obliquely views, via the pentaprism, the object image formedon the focus plate 106. An AF mirror 110 guides, to an AF sensor 111 ina focus detection circuit, some components of a light beam that hasentered from the photographing lens 200 and passed through the halfmirror 105. The focus detection circuit performs focus detection usingthe light beam. An LCPU (to be referred to as a lens microcomputerhereinafter) 201 is a microcomputer in the lens, and sends distanceinformation with respect to the object to the camera microcomputer 101.

The arrangement of the flash 300 will be described next. An SCPU (to bereferred to as a flash microcomputer hereinafter) 301 is a microcomputerthat controls each unit of the flash 300. A light amount control device302 includes a boost circuit configured to boost a battery voltage toturn on a light source 305 (to be described later), and a currentcontrol circuit configured to control the start and stop of lightemission. A zoom optical system 303 includes a panel such as a Fresnellens and changes the irradiation angle of the flash 300. A reflector 304condenses a light beam emitted by the light source 305 and irradiatesthe object with the light beam. The light source 305 is formed from axenon tube or white LED. The arrangement of the image capturingapparatus shown in FIG. 1 has been described above.

The operation of the camera body 100 will be described next withreference to the flowcharts shown in FIGS. 2 and 3. Note that it isassumed here that the camera body 100 is powered on and set in an imagecapturing standby state in the initial state.

In step S101, the camera microcomputer 101 determines whether the firststroke (to be referred to as SW1 hereinafter) of the shutter switch isturned on. If the shutter switch SW1 is on, the process advances to stepS102. In step S102, the camera microcomputer 101 drives the photometricsensor 107 and performs various kinds of calculations such asphotometry. Note that details of the various kinds of calculations willbe described later with reference to FIG. 3.

In step S103, the camera microcomputer 101 performs AF (Auto Focus)processing of a known phase difference method. The camera microcomputer101 detects the defocus amount, drives the focus lens of thephotographing lens 200 via the lens microcomputer 201, and drives thefocus lens by an amount corresponding to the detected defocus amount.

In step S104, the camera microcomputer 101 determines whether the secondstroke (to be referred to as SW2 hereinafter) of the shutter switch isturned on. If the shutter switch SW2 is off, the camera microcomputer101 confirms the state of the shutter switch SW1 in step S105. If theshutter switch SW1 remains on, the process returns to step S102. If theshutter switch SW1 is turned off, the process returns to step S101.

If the shutter switch SW2 is on in step S104, in step S106, the cameramicrocomputer 101 executes shooting processing based on exposure controlvalues calculated by the photometric processing of step S102.

FIG. 3 is a flowchart showing details of the photometric processing ofstep S102 in FIG. 2. The flowchart of FIG. 3 will be described withreference to FIGS. 4A to 9.

FIG. 4A is a view showing an example of a shooting scene. As forbrightness represented by a BV value in the APEX unit, the mountainportion has BV 1, the face portion has BV 9, the sky portion has BV 12,and the sun portion has BV 20.

In step S201, the camera microcomputer 101 decides the accumulation time(TV) of the photometric sensor 107, performs photometry accumulationprocessing, generates image data, and saves it in the memory 102. Theimage data is assumed to be a 24-bit high dynamic range image afteroptical correction by a lens and the like. Note that the opticalcorrection method of the lens and the like is not directly relevant tothe embodiment, and a detailed description thereof will be omitted. As amethod of generating an image (high dynamic range image) with anexpanded dynamic range, a method of capturing a plurality of imagesunder different exposure values by a plurality of times of exposure andcombining them can be considered. A method of forming each pixel of thephotometric sensor by at least two types of pixel regions, that is, alow sensitivity pixel region and a high sensitivity pixel region andgenerating an image with an expanded dynamic range by one exposure isalso considerable. As a method of capturing a plurality of images andcombining them, a method of weighting and adding the pixel outputs of aplurality of images on a pixel basis or a method of selecting the pixeloutputs of a plurality of images on a pixel basis in accordance with thebrightness of the object can be considered. As the method of formingeach pixel of the photometric sensor by two types of pixel regions andthus generating an image with an expanded dynamic range as well, amethod of weighting and adding the outputs of the two types of pixelregions on a pixel basis or a method of selecting the output of one ofthe pixel regions on a pixel basis in accordance with the brightness ofthe object can be considered. However, the method of generating an imagewith an expanded dynamic range is not limited to these methods. In thisembodiment, for example, the method of forming each pixel of thephotometric sensor 107 by two types of pixel regions and selecting theoutput of one of the pixel regions on a pixel basis in accordance withthe brightness of the object is used. The sensitivity of the lowsensitivity pixel region and the high sensitivity pixel region of eachpixel of the photometric sensor 107 are set so as to enable photometryfrom BV −7 to BV 16, as shown in FIG. 5.

FIG. 5 is a correspondence table of BV values and the pixel outputs ofthe 24-bit high dynamic range image. As is apparent from FIG. 5, thephotometric sensor 107 can perform photometry from BV −7 to BV 16,covering the photometry range (BV −5 to BV 15) required of the imagecapturing apparatus. The pixel outputs are 0 to 16,777,215.

FIG. 4B is a view showing the relationship between the shooting sceneand the pixels of the photometric sensor 107. The photometric sensor 107is assumed to have 36 horizontal pixels×24 vertical pixels=864 pixels.FIGS. 4D and 4E show partial pixel outputs in the thick frames of theimage data shown in FIG. 4C.

For example, as can be seen from FIGS. 4A and 6, the pixel output of themountain portion (BV 1) is 255, the pixel output of the face portion (BV9) is 65,535, and the pixel output of the sky portion (BV 12) is524,287. The pixel output of the sun portion (BV 20) is 16,777,215corresponding to BV 17. The output is saturated, and photometry cannotcorrectly be performed. However, the value falls outside the photometryrange (BV −5 to BV 15) required of the image capturing apparatus, andprocessing can be performed without any problem in object recognition ofstep S205 and brightness averaging of step S206 to be described later.

In the next calculations of steps S202 to S207, 14-bit data (0 to16,383) of a predetermined data amount is extracted from the 24-bit highdynamic range image acquired in step S201, and various kinds ofcalculations are performed. To do photometry within the photometry range(BV −5 to BV 15) required of the image capturing apparatus, a dynamicrange of 20 steps is necessary. In many scenes, however, the brightnessdifference in a screen is 14 steps or less. For this reason, appropriateprocessing can be executed by extracting appropriate 14-bit data(compressed data) in correspondence with 24-bit data of a linear value.

The BV value is a log (logarithm) value. To the contrary, a pixel outputis a linear value and changes in powers of 2. That is, anincrease/decrease of one step in the log value corresponds to anincrease/decrease of one bit in the linear value. In addition, anincrease/decrease of one bit in the linear value can be expressed by bitshift. That is, extraction of 14 bits corresponding to 24 bits isexpressed by bit shift.

For example, to extract 14 steps from BV 0 to BV 14 (pixel outputs of 64to 2,097,151), the pixel outputs are converted into 0 to 16,383 by 6-bitshift. However, a pixel output of 2,097,152 or more still includes 14bits even after bit shift and need therefore be clipped to 16,383.

In addition, object recognition of step S205 and brightness averaging ofstep S206 to be described later are performed using 14-bit data obtainedby compressing 24-bit data, thereby decreasing the calculation amount.This can reduce the circuit scale and shorten the processing time.

A method of deciding an appropriate bit shift amount will be describedbelow concerning steps S202 and S203. In step S202, the cameramicrocomputer 101 generates the histograms of the most significant bitsof pixel outputs of the image data.

FIG. 6 is a view showing the corresponding relationship between thepixel outputs of the photometric sensor 107 and most significant bits.FIGS. 4F and 4G show the most significant bits of the pixel outputs inFIGS. 4D and 4E, respectively. The most significant bits of the mountainportion (255) are 8 bits, the most significant bits in the face portion(65,535) are 16 bits, the most significant bits in the sky portion(524,287) are 18 bits, and the most significant bits in the sun portion(16,777,215) are 24 bits, which are counted as the most significantbits. The histograms of most significant bits as shown in FIG. 7 aregenerated for the image data shown in FIG. 4A.

In step S203, the camera microcomputer 101 decides the bit shift amountof the image from the histograms calculated in step S202. An example ofcalculation of deciding the bit shift amount will be described.

First, the cumulative histograms from the upper bits of the histogramsare calculated. FIG. 8 shows the cumulative histograms from the upperbits of the histograms shown in FIG. 7. Next, most significant bitswhose cumulative histogram exceeds a predetermined threshold areobtained. In the example of FIG. 8, when the threshold is 20%, the mostsignificant bits (to be referred to as most significant bits more thanthe threshold hereinafter) whose cumulative histogram exceeds thethreshold are 18 bits.

Next, a bit shift amount by which the most significant bits more thanthe threshold become a predetermined output when the 24-bit data iscompressed into a predetermined data amount is decided. Here, an examplein which 24-bit data (0 to 16,777,215) is compressed into 14-bit data (0to 16,383) will be explained. A bit shift amount by which apredetermined output of 13 bits is obtained after the most significantbits more than the threshold, that is, 18 bits of 24-bit data arecompressed into 14-bit data is decided.

The bit shift amount is obtained by

(bit shift amount)=Max{(most significant bits more thanthreshold)−(predetermined output bits), 0}  (1)

From equation (1), the bit shift amount is 5 bits.

In step S204, the camera microcomputer 101 compresses the image datainto the predetermined data amount using the bit shift amount decided instep S203. However, if the data amount exceeds the predetermined dataamount even after bit shift, the resultant data is clipped to thepredetermined data amount.

FIG. 9 shows an example in which 24-bit data (0 to 16,777,215) iscompressed into 14-bit data (0 to 16,383) of the predetermined dataamount. The abscissa represents the pixel output and most significantbits, and the ordinate represents the pixel output and most significantbits after bit shift processing and clip processing are performed instep S204. The bit shift amount is 5 bits. FIGS. 4H and 4I show outputsobtained by shifting the data shown in FIGS. 4D and 4E by 5 bits,respectively.

As is apparent from FIGS. 9 and 4A to 4J, when the pixel output of65,535 (most significant bits are 16 bits) in the face portion isshifted by 5 bits, 2,047 is obtained. When the pixel output of16,777,215 (most significant bits are 24 bits) is shifted by 5 bits,524,287 is obtained. This falls outside the range of the predetermineddata amount of 14 bits (0 to 16,383) and is therefore clipped to 16,383.

In step S205, the camera microcomputer 101 performs known objectrecognition processing using the image data of the predetermined dataamount generated in step S204. In this object recognition processing,light source determination processing (AWB) of determining the lightsource for illuminating the object, feature color extraction processingof extracting a feature color such as a flesh color, tracking processingof tracking the object by a method such as block matching, facerecognition processing of extracting a feature region such as a face,and the like are performed. Note that the above-described processesexecuted in the object recognition processing are not directly relevantto the present invention, and a detailed description thereof will beomitted.

In step S206, the camera microcomputer 101 performs brightness averagingcalculation using the image data of the predetermined data amountgenerated in step S204. For example, the pixels of the photometricsensor 107 are divided into 6 horizontal areas×4 vertical areas=24 areaseach corresponding to one block including 6 vertical pixels×6 horizontalpixels, as shown in FIG. 4J. A photometric output value Y of each areais calculated using the 14-bit image data calculated in step S204.

A weighted average value Yw of the photometric output value Y of eacharea and an exposure control value weighting coefficient k (to bedescribed later) is calculated by

Yw=ΣYij×kij  (2)

where Yij and kij represent the photometric output value Y and theexposure control value weighting coefficient k of each area,respectively, and i is the area number in the horizontal direction and jis the area number in the vertical direction. The number of additionschanges depending on the number of area divisions.

The exposure control value weighting coefficient k is a coefficient usedto change the weighting of the photometric output value of eachphotometry area in accordance with the image capturing mode and thephotometry mode of the camera body 100 or a shooting scene. For example,if the photometry mode is a center photometry mode, weightingcoefficients in the photometry areas near the center of the image areset to be larger than those on the periphery of the image. In addition,if the image capturing apparatus has a feature region detectionfunction, and an image capturing mode using the feature region detectionfunction is set, weighting coefficients in photometry areascorresponding to feature regions are set to be larger than those in theother photometry areas.

If the image capturing apparatus has a scene determination function ofautomatically determining the type of a shooting scene in accordancewith the state of the field, weighting coefficients optimum for thedetermined scene are set for the photometry areas. The exposure controlvalue weighting coefficient k is not directly relevant to theembodiment, and a more detailed description thereof will be omitted.

In step S207, the camera microcomputer 101 converts the bit shift amountaccording to Yw calculated in step S204. In the example of FIGS. 4A to4J, Yw is multiplied by 2⁵ of the bit shift amount of 5 bits and thusreturned to a value corresponding to the output of 24-bit data.

The camera microcomputer 101 calculates the exposure control values (forexample, time value, aperture value, and film speed value) for finalshooting based on the object brightness (photometric value) obtainedfrom the accumulation time and the weighted average value Yw that hasundergone conversion processing according to the bit shift amount. Notethat since a method of deciding the exposure control values is notdirectly relevant to the embodiment, and an arbitrary method can beemployed, a detailed description thereof will be omitted. For example, amethod of obtaining the exposure control values for final shooting basedon the obtained photometric value and a program diagram stored in thememory 102 in advance is usable.

With the above-described method, it is possible to perform appropriatephotometric calculation processing while compressing various kinds ofcalculation amounts. The first embodiment has been described above.

Second Embodiment

The flash light-emitting amount decision method of a camera according tothe second embodiment of the present invention will be described next.Note that the arrangement of an image capturing apparatus according tothe second embodiment is the same as the arrangement of the camera shownin FIG. 1.

The operations of a camera body 100 and a flash 300 will be describedwith reference to the flowchart of FIG. 10. Note that only the operationin flash photography will be explained here.

In step S301, a camera microcomputer 101 generates image dataimmediately before pre-light emitting and holds it in a memory 102. Instep S302, the camera microcomputer 101 performs pre-light emitting,generates image data at the time of pre-light emitting, and holds it inthe memory 102.

In step S303, the camera microcomputer 101 obtains reflected light imagedata by subtracting the image data before pre-light emitting from theimage data at the time of pre-light emitting. Image data of only flashlight excluding the influence of outside light can thus be obtained.This image data is held in the memory 102.

In step S304, the camera microcomputer 101 generates the histograms ofthe most significant bits of the image data acquired in step S302 orS303. This processing is the same as that of step S202 of the firstembodiment, and a detailed description thereof will be omitted.

In step S305, the camera microcomputer 101 decides the bit shift amountof the image from the histograms calculated in step S304. Thisprocessing is the same as that of step S203 of the first embodiment, anda detailed description thereof will be omitted. However, unlike thefirst embodiment, when deciding the light-emitting amount of the flash,it is necessary to accurately detect a reflected light from an object ata short distance. Hence, the threshold is set lower than in AEprocessing. This suppresses saturation of the reflected light from theobject at the short distance in the image after bit shift.

In step S306, the camera microcomputer 101 compresses the image dataacquired in steps S301, S302, and S303 into a predetermined data amountusing the bit shift amount decided in step S304. However, if the dataamount exceeds the predetermined data amount even after bit shift, theresultant data is clipped to the predetermined data amount. Thisprocessing is the same as that of step S204 of the first embodiment, anda detailed description thereof will be omitted.

In step S307, the camera microcomputer 101 performs known objectrecognition processing using the image generated in step S306. In thisobject recognition processing, feature color extraction processing ofextracting a feature color such as a flesh color, face recognitionprocessing of extracting a feature region such as a face, and the likeare performed. Note that the above-described processes executed in theobject recognition processing are not directly relevant to the presentinvention, and a detailed description thereof will be omitted.

In step S308, the camera microcomputer 101 performs reflected lightaveraging calculation using the image data generated in step S306. As inthe first embodiment, a weighted average value Ys of a photometricoutput value Y of each area and a light-emitting amount control valueweighting coefficient ks (to be described later) is calculated. Thisprocessing is the same as that of step S206 of the first embodiment, anda detailed description thereof will be omitted.

In step S309, the camera microcomputer 101 converts the bit shift amountaccording to Ys calculated in step S308. This processing is the same asthat of step S207 of the first embodiment, and a detailed descriptionthereof will be omitted.

In step S310, the camera microcomputer 101 performs logarithmicconversion of the pre-light emitting reflected light brightness value Ysthat has undergone conversion processing according to the bit shiftamount based on a logarithmic conversion table prepared in the memory102 in advance, and obtains a pre-light emitting reflected lightbrightness value Yslog after logarithmic conversion. A difference DFfrom an appropriate brightness value Yt (logarithm), that is,DF=Yslog−Yt is obtained from the resultant pre-light emitting reflectedlight brightness value Yslog. A light-emitting amount ANSWER of finallight emission is decided from the difference DF (the step differencebetween the brightness in pre-light emitting and the appropriatebrightness) and the pre-light emitting amount.

ANSWER=(pre-light emitting amount)+DF  (4)

The light-emitting amount ANSWER of final light emission is sent to thecamera microcomputer 101, and the light-emitting amount is sent from thecamera microcomputer 101 to a flash microcomputer 301.

Finally in step S311, the camera microcomputer 101 issues a lightemission instruction to the flash microcomputer 301, and the flashmicrocomputer 301 controls a light amount control device 302 to executefinal light emission and perform final shooting. The second embodimenthas been described above.

With the above-described method, it is possible to perform appropriatephotometric calculation processing while compressing various kinds ofcalculation amounts even in a state in which the object brightnesswidely ranges from a low brightness to a high brightness.

Two preferred embodiments of the present invention have been describedabove. The invention is not limited to the embodiments, and variouschanges and modifications can be made within the spirit and scope of thepresent invention. For example, in the two embodiments described above,an example of an exposure control apparatus having an image capturingfunction like an image capturing apparatus has been described. However,it may be an exposure control apparatus without the image capturingfunction. In the exposure control apparatus without the image capturingfunction, exposure control values are calculated based on image datainput from an external device.

Other Embodiments

Embodiment(s) of the present invention can also be realized by acomputer of a system or apparatus that reads out and executes computerexecutable instructions (e.g., one or more programs) recorded on astorage medium (which may also be referred to more fully as a‘non-transitory computer-readable storage medium’) to perform thefunctions of one or more of the above-described embodiment(s) and/orthat includes one or more circuits (e.g., application specificintegrated circuit (ASIC)) for performing the functions of one or moreof the above-described embodiment(s), and by a method performed by thecomputer of the system or apparatus by, for example, reading out andexecuting the computer executable instructions from the storage mediumto perform the functions of one or more of the above-describedembodiment(s) and/or controlling the one or more circuits to perform thefunctions of one or more of the above-described embodiment(s). Thecomputer may comprise one or more processors (e.g., central processingunit (CPU), micro processing unit (MPU)) and may include a network ofseparate computers or separate processors to read out and execute thecomputer executable instructions. The computer executable instructionsmay be provided to the computer, for example, from a network or thestorage medium. The storage medium may include, for example, one or moreof a hard disk, a random-access memory (RAM), a read only memory (ROM),a storage of distributed computing systems, an optical disk (such as acompact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™),a flash memory device, a memory card, and the like.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2014-213982, filed Oct. 20, 2014, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. An exposure control apparatus comprising: anacquisition unit including a plurality of pixels that are arrangedtwo-dimensionally, and configured to acquire image data; a compressionunit configured to compress the image data and generate compressed data;a calculation unit configured to calculate a first photometric valuebased on the compressed data; a conversion unit configured to convertthe first photometric value into a second photometric valuecorresponding to the image data before the compression; and an exposurecontrol unit configured to perform exposure control based on the secondphotometric value.
 2. The apparatus according to claim 1, wherein theimage data is image data that has undergone processing for expanding adynamic range as compared to a dynamic range of said acquisition unit.3. The apparatus according to claim 1, wherein said compression unitcompresses the image data using bit shift processing and clipprocessing.
 4. The apparatus according to claim 3, wherein saidconversion unit converts the first photometric value into the secondphotometric value by adding an amount processed by the bit shiftprocessing to the first photometric value.
 5. The apparatus according toclaim 3, wherein said compression unit decides a shift amount of the bitshift processing based on a histogram of a most significant bit of theimage data.
 6. The apparatus according to claim 5, wherein saidcompression unit sets the shift amount of the bit shift processing suchthat the most significant bit whose cumulative histogram from an upperbit of the image data exceeds a predetermined threshold becomes apredetermined output.
 7. The apparatus according to claim 6, whereinsaid compression unit changes the predetermined threshold in accordancewith a shooting mode.
 8. The apparatus according to claim 1, whereinsaid calculation unit divides the plurality of pixels of saidacquisition unit into a plurality of regions, and weights the compresseddata for each of the plurality of regions, thereby calculating the firstphotometric value.
 9. The apparatus according to claim 1, wherein saidacquisition unit acquires the image data after optical correction of aphotographing lens.
 10. An image capturing apparatus comprising: anacquisition unit including a plurality of pixels that are arrangedtwo-dimensionally, and configured to acquire image data; a compressionunit configured to compress the image data and generate compressed data;a calculation unit configured to calculate a first photometric valuebased on the compressed data; a conversion unit configured to convertthe first photometric value into a second photometric valuecorresponding to the image data before the compression; an exposurecontrol unit configured to perform exposure control based on the secondphotometric value; and an image capturing unit configured to capture anobject image under exposure control by said exposure control unit.
 11. Acontrol method of an exposure control apparatus including an acquisitionunit including a plurality of pixels that are arrangedtwo-dimensionally, and configured to acquire image data, comprising:compressing the image data and generating compressed data; calculating afirst photometric value based on the compressed data; converting thefirst photometric value into a second photometric value corresponding tothe image data before the compression; and performing exposure controlbased on the second photometric value.
 12. A computer-readable storagemedium storing a program that causes a computer to execute each step ofa control method of an exposure control apparatus including anacquisition unit including a plurality of pixels that are arrangedtwo-dimensionally, and configured to acquire image data, the controlmethod comprising: compressing the image data and generating compresseddata; calculating a first photometric value based on the compresseddata; converting the first photometric value into a second photometricvalue corresponding to the image data before the compression; andperforming exposure control based on the second photometric value.